Some cars were parked in the hardware store parking lot. 8 more cars park at the hardware store. Now there are 39 cars in the parking lot. How many cars were parked at the hardware store at the beginning?
Explanation: The number of cars parked at the store at the beginning is the difference of the number parked now and the number that came and parked earlier. The difference is $39 - 8$ cars. $39 - 8 = 31$ cars.